1. Field of the Invention
The present invention relates to entrapping immobilization pellets and a process for producing the same, and wastewater treatment process and equipment using the same. More particularly, the present invention relates to entrapping immobilization pellets that biologically denitrify ammonia from wastewater containing ammonia and a process for producing the same, and wastewater treatment process and equipment using the same.
2. Description of the Related Art
Nitrifying bacteria contained in activated sludge used for wastewater treatment grow slower than common bacteria. In particular, in winter during which water temperatures are low, such nitrifying bacteria have only a small number of bacterial cells and thus exhibit significantly reduced nitrification activity. This also applies to microorganisms having the same properties as in nitrifying bacteria.
In this situation, wastewater nitrification performance of nitrifying bacteria has been improved by immobilizing activated sludge containing nitrifying bacteria to the surface of an attachment material such as quartz sand, activated carbon, or plastic to increase the concentration of nitrifying bacteria (reference: Water Treatment by Microorganism Immobilization Process: Pellet Immobilization Process, Entrapping Immobilization Process, Biologically Activated Carbon Process, published by NTS Inc. in 2000).
However, the concentration of nitrifying bacteria cannot be sufficiently increased in attachment immobilization pellets in which microorganisms are attached to an attachment material, because the attached microorganisms are released from the material, or microorganisms differing from nitrifying bacteria as target microorganisms are attached to the material. Accordingly, wastewater has been treated at high speed with increased nitrification activity by producing entrapping immobilization pellets in which useful microorganisms such as nitrifying bacteria are entrapped and immobilized in an immobilizing material, and packing a reaction tank with the entrapping immobilization pellets to increase the concentration of nitrifying bacteria.
It is assumed that entrapping immobilization pellets have higher nitrification activity and can exhibit pellet performance earlier as the pellets have a larger number of nitrifying bacteria at the initial stage. Therefore, activated sludge containing nitrifying bacteria has conventionally been concentrated and then entrapped and immobilized in order to increase the number of nitrifying bacteria at the initial stage (reference: Research Process for Environmental Microbial Engineering, p. 123, Gihodo Shuppan Co., Ltd., published in 1993). Activated sludge refers to flocculated microorganisms grown by bubbling air to organic wastewater such as sewage, and the microorganisms include bacteria, fungi, protozoa, and micrometazoa. Organic substances in wastewater are adsorbed by bacteria and then taken and assimilated in the body, so that the bacteria are grown. The ecosystem is stable by the food chain in which the bacteria are through protozoa, micrometazoa, etc. (reference: Technology Seminar Text, The Actuality of Measures against Various Troubles in the Activated Sludge Process, Technical Information Center, Co., Ltd., July 2000). Activated sludge generally contains a large amount of organic substances in this manner.
Apart from entrapping immobilization pellets, there has been proposed a process for producing pellets having a large surface area on which useful microorganisms are supported (for example, Japanese Patent No. 3131678). The pellets are obtained by granulating purified water sludge mainly composed of inorganic components and then drying and burning the granules, and are supposed to result in porous microbial pellets having an extremely large specific area. There has also been proposed to mix sludge with useful microorganisms or cause microbial pellets to support useful microorganisms.